how to resize an image so it shows in its entirety within an object?
UV Unwrapping

The best thing to do would be to start with a blank cube, then select only the face that you want to have the car. Key Numpad 5 to enter Orthographic View, and then key either Numpad 1, Numpad 3, Numpad 7, Ctrl + Numpad 1, Ctrl + Numpad 3, or Ctrl + Numpad 7 (whichever makes you face the selected face). Key U and select Project from View (Bounds).



Node-based Mapping



The .blend file for the second result: